As a link mechanism of an internal combustion engine, conventionally known is a variable compression ratio mechanism of the type disclosed in Patent Document 1.
This variable compression ratio mechanism utilizes a multi-link type piston-crank system so as to change the stroke characteristics of each piston and thereby vary the actual mechanical compression ratio of the internal combustion engine.
More specifically, the piston is connected to a crankshaft via upper and lower links. The attitude of the lower link is controlled by means of an actuator. The actual compression ratio of the internal combustion engine is varied by changing the stroke characteristics of the piston according to the attitude of the lower link.
The actuator includes: a housing; a reduction gear unit and a drive motor, both of which are mounted to an outer side of the housing; a control shaft (as a second control shaft in the above-mentioned patent document) rotatably supported by the housing and adapted to receive a torque from through the reduction gear unit; an eccentric shaft part (as a second eccentric shaft part in the above-mentioned patent document) integrally formed on a tip end portion of the control shaft; and a connection link having one end portion connected to the lower link and the other end portion connected to an eccentric shaft part of a control shaft that extends substantially in parallel with the crankshaft.
When the rotation position of the control shaft is shifted by the torque outputted from the drive motor and the reduction gear unit, the attitude of the lower link is controlled by such positional shift of the control shaft through the eccentric shaft part and the connection link.